1. Field of the Invention
The present invention is related to a direct current converter, and more particularly, to a direct current converter capable of preventing body diodes from conducting and accurately detecting operation faults.
2. Description of the Prior Art
An electronic device includes various components, each of which may operate in a different voltage level. Therefore, a DC-DC (direct current to direct current) voltage converter is definitely required to adjust (step up or down) and stabilize the voltage level in the electronic device. Originating from a buck (or step down) converter and a boost (or step up) converter, various types of DC-DC voltage converters are accordingly customized to meet different power requirements. As implied by the names, the buck converter is utilized for stepping down a DC voltage of an input terminal to a default voltage level, and the boost converter is for stepping up the DC voltage of the input terminal. With the advancement of modern electronics technology, both of the buck converter and the boost converter are modified and customized to conform to different architectures or to meet different requirements.
For example, please refer to FIG. 1, which is a schematic diagram of a bootstrap buck converter 10 of the prior art. The bootstrap buck converter 10 comprises a driving-stage circuit 100, an output-stage circuit 102, a control module 104 and a bootstrap circuit 106. In the bootstrap buck converter 10, an input terminal provides a fixed voltage Vin. By using the control module 104 and the bootstrap circuit 106 generate control signals to the driving-stage circuit 100, in order to exchange energy via an inductor in the output-stage circuit 102, so as to acquire a stable DC voltage source Vo with a magnitude less than the voltage Vin of the input terminal. The driving-stage circuit 100 comprises power transistors Q1, Q2 and driving units DRV_1, DRV_2. Through the driving units DRV_1, DRV_2 and the bootstrap circuit 106, the bootstrap buck converter 10 can controls the power transistors Q1, Q2 according to control signals V_CTRL, V_CTRL_B generated by the control module 104, and output a switching signal from a node Y to the output-stage circuit 102. The output-stage circuit 102 comprises an inductor L, a capacitor C and feedback resistors R1, R2. Via the inductor L, the switching signal of the node Y can exchange energy with the output terminal Vo, and the capacitor C can stabilize the voltage of the output terminal Vo and alleviate voltage variations at the output terminal. The voltage of the output terminal Vo can feedback a voltage VFB through the feedback resistors R1, R2, such that the control module 104 can accordingly generate the control signals V_CTRL, V_CTRL_B. The bootstrap circuit 106 comprises a bootstrap capacitor C_BS and a diode D_BS. Operations of the bootstrap buck converter 10 are well known to those skilled in the art, i.e. the control module 104 and the bootstrap circuit 106 generate driving signals to enable the power transistor Q1 and disable the power transistor Q2, or swap over, so as to keep the inductor L operating between a charge status and a discharge status. In such a situation, through the control signal V_CTRL, the control module 104 can adjust a switching frequency between the charge status and the discharge status according to the feedback signal VFB generated by the feedback resistors R1, R2, to generate the desired output voltage Vo.
In the bootstrap buck converter 10, preferably, the driving units DRV_1, DRV_2 are inverters, driven by node voltages V_X, V_Y of nodes X and Y, the input voltage Vin and a low-potential voltage Vss. The node X is located between the bootstrap capacitor C_BS and the diode D_BS, and the node Y is located among the driving-stage circuit 100, the output-stage circuit 102 and the bootstrap circuit 106. When the power transistor Q2 is enabled, the node voltage V_Y approaches the low-potential voltage Vss, and a bootstrap voltage Vcc charges the bootstrap capacitor C_BS via the diode D_BS. On the contrary, when the power transistor Q2 is disabled, the node voltage V_Y transiently increases to approach the input voltage Vin. Meanwhile, since energy stored in the bootstrap capacitor C_BS is not exhausted, the node voltage V_X would increase to a magnitude equal to the node voltage V_Y plus the bootstrap source terminal voltage Vcc, great enough to activate the driving unit DRV_1. Thus, when the bootstrap buck converter 10 switches from the mode “Q1: disable; Q2: enable” to the mode “Q1: enable; Q2: disable”, the bootstrap capacitor C_BS can transiently increase the node voltage V_X by the bootstrap circuit 106, to activate the driving unit DRV_1.
In the prior art, the bootstrap circuit 106 are preferably positioned outside a chip. Taking the voltage bearing ability of the diode D_BS into consideration, manufacturing cost and circuit complexity would accordingly increase. In order to overcome this disadvantage, the prior art has developed an architecture replacing or simulating the diode D_BS of the bootstrap circuit 106 by a metal oxide semiconductor field effect transistor (MOSFET), so as to embed the MOSFET in the chip. Even though the MOSFET functions almost the same as the diode D_BS, in some cases, a leakage current between the node X and the bootstrap voltage terminal probably follows though a parasitical body diode embedded within the MOSFET, and leads to irregularities and poorer efficiency in the bootstrap circuit 106 and thereby a negative influence on the whole functionality of the bootstrap buck converter 10. In addition, in the bootstrap buck converter 10, once the node X may be short-circuit due to component breakdown issues, the external high-voltage terminal (the voltage Vcc) would be pulled low, which negatively affects inner operations of the chip.
Therefore, enhancing the bootstrap circuit in the buck converter has been one of the objectives and the industry is focusing on.